DeoxyriboNucleic Acid; the molecule that stores genetic information in cells, organized into genes and chromosomes.

A segment of DNA that contains instructions for making a specific protein or functional RNA, influencing a trait.

A long, tightly coiled piece of DNA wrapped around proteins. Humans usually have 46 in body cells, arranged in 23 pairs.

A different version of the same gene, such as a brown-eye allele vs. a blue-eye allele.

The combination of alleles an organism has for a gene, often written as letter pairs (for example, BB, Bb, or bb).

DeoxyriboNucleic Acid; the molecule that stores genetic information in cells.

A segment of DNA that codes for a specific protein or functional RNA, influencing a trait.

A different version of the same gene, often represented by different letters.

Genetics Basics: DNA, Genes, and Inheritance Patterns — SHS Science Foundations: Integrated Chemistry, Biology, and Physics

Q: A plant has the genotype `Tt` for height, where `T` (tall) is dominant over `t` (short). Which statement is correct?

Its genotype is `Tt` and its phenotype is tall.. `Tt` describes the **genotype** (the alleles). Because `T` is dominant, the plant's **phenotype** is tall. Phenotype is described with words like 'tall' or 'short', not with allele letters.

Q: In a simple dominant–recessive trait, which genotype will always show the recessive phenotype?

hh. The recessive phenotype appears only when **both** alleles are recessive, written as `hh`. Any genotype with a dominant allele (H) will show the dominant phenotype.

From Cells to DNA: Where Heredity Lives

Zooming In

You already know cells are the basic units of life. Now we zoom in further to see how cells store and pass on information from one generation to the next.

Meet DNA

Almost every cell in your body contains DNA (DeoxyriboNucleic Acid), a chemical instruction book that tells cells how to build and run the organism.

Where DNA Lives

In eukaryotic cells, DNA is stored mainly in the nucleus, the control room of the cell. There, DNA acts like a recipe book for making proteins.

DNA and Heredity

When organisms reproduce, they pass copies of their DNA to offspring. That is how traits like eye color, blood type, and some disease risks are inherited.

Factory Analogy

Think of the cell as a factory, the nucleus as the control room, and DNA as the master instruction manual that keeps everything running correctly.

DNA, Genes, and Chromosomes: Organizing the Code

DNA Structure

DNA is a long molecule shaped like a double helix, a twisted ladder. The sides are sugar and phosphate; the rungs are base pairs A-T and C-G.

Base Pair Rules

In DNA, A (adenine) always pairs with T (thymine), and C (cytosine) always pairs with G (guanine). This pairing makes copying DNA very reliable.

What is a Gene?

A gene is a specific segment of DNA that contains instructions for making one protein or functional RNA. Each gene is like a single recipe in a cookbook.

What are Chromosomes?

Chromosomes are tightly coiled DNA wrapped around proteins. Each chromosome contains many genes lined up along its length.

Human Chromosomes

Most human body cells have 46 chromosomes in 23 pairs: 22 pairs of autosomes and 1 pair of sex chromosomes (XX or XY in most humans).

Passing Chromosomes On

During reproduction, parents pass on one chromosome from each pair to their offspring. This is how children get half their genetic material from each parent.

Alleles, Genotype, and Phenotype

What are Alleles?

An allele is a different version of the same gene. For example, a gene for eye color might have a brown-eye allele and a blue-eye allele.

Genotype

Your genotype is the combination of alleles you have. For a single gene we often write this as two letters, like `BB`, `Bb`, or `bb`.

Phenotype

Your phenotype is the observable trait that results from your genotype, such as brown eyes, blue eyes, blood type A, or curly hair.

Code vs. Appearance

Genotype is the genetic code; phenotype is how that code shows up in your body. Different genotypes can sometimes produce the same phenotype.

Dominant and Recessive Alleles

Dominant vs. Recessive

A dominant allele shows its effect if you have just one copy. A recessive allele only shows its effect when you have two copies and no dominant allele.

Letter Notation

We use capital letters for dominant alleles (like `B`) and lowercase for recessive alleles (like `b`). A genotype has two letters, one from each parent.

Genotype Types

`BB` is homozygous dominant, `Bb` is heterozygous, and `bb` is homozygous recessive. These genotypes can lead to different phenotypes.

Eye Color Example

If brown (B) is dominant over blue (b), both `BB` and `Bb` give brown eyes, while only `bb` gives blue eyes. Many real traits are more complex, but this model is useful.

Real-World Traits: Earlobes and Blood Type

Earlobe Example

Imagine `F` = free earlobes (dominant) and `f` = attached (recessive). `FF` and `Ff` give free earlobes; only `ff` gives attached earlobes.

Parents with Earlobes

If one parent is `Ff` (free) and the other is `ff` (attached), children may have free or attached earlobes depending on which alleles they inherit.

ABO Blood Type Overview

ABO blood type uses three alleles: `IA`, `IB`, and `i`. `IA` and `IB` are dominant over `i` and are codominant with each other.

Blood Type Genotypes

`IAIA` or `IAi` = Type A, `IBIB` or `IBi` = Type B, `IAIB` = Type AB, `ii` = Type O. This shows codominance and multiple alleles.

Why This Matters

Earlobes fit a simple dominant–recessive model. ABO blood type reminds us that real genetics can be more complex, with multiple alleles and codominance.

Monohybrid Crosses and Punnett Squares

Monohybrid Cross

A monohybrid cross tracks how one gene with two alleles is passed from parents to offspring. It focuses on a single trait at a time.

Punnett Square Purpose

A Punnett square is a grid that helps you predict possible offspring genotypes and phenotypes based on the parents' genotypes.

Steps 1–3

1) Choose a trait and letter alleles. 2) Write parent genotypes. 3) Work out possible gametes (egg or sperm) each parent can produce.

Steps 4–7

4) Draw a 2×2 grid. 5) Put one parent's gametes on top and the other's on the side. 6) Fill boxes by combining alleles. 7) Count genotype and phenotype ratios.

Key Assumption

This method assumes each parent contributes one allele per gene, and alleles separate into gametes randomly, following Mendel's law of segregation.

Build a Punnett Square: Step-by-Step

Use this guided exercise to practice a monohybrid cross.

Trait: Freckles

`F` = freckles (dominant)
`f` = no freckles (recessive)

Parents:

Parent 1: `Ff` (has freckles)
Parent 2: `Ff` (has freckles)

Work through these steps mentally or on paper:

List the alleles each parent can pass on.

Parent 1 (Ff) gametes: `F` or `f`
Parent 2 (Ff) gametes: `F` or `f`

Draw a 2×2 Punnett square on your paper.

Write Parent 1’s gametes across the top: `F` and `f`.
Write Parent 2’s gametes down the left side: `F` and `f`.

Fill in the four boxes by combining the alleles from the top and side of each box.

Top-left box: `F` (from top) + `F` (from side) = `FF`
Top-right box: `f` + `F` = `Ff`
Bottom-left box: `F` + `f` = `Ff`
Bottom-right box: `f` + `f` = `ff`

Count genotypes.

`FF`: 1
`Ff`: 2
`ff`: 1
Genotype ratio: 1 `FF` : 2 `Ff` : 1 `ff`

Convert to phenotypes.

`FF` → freckles
`Ff` → freckles
`ff` → no freckles
Phenotype ratio: 3 freckles : 1 no freckles

Think: If these parents have many children, about 3 out of 4 would be expected to have freckles, and 1 out of 4 to have no freckles. Real families are small, so actual numbers can differ, but the probabilities stay the same.

Check Understanding: Genotype vs. Phenotype

Answer this question to test your understanding of genotype and phenotype.

A plant has the genotype `Tt` for height, where `T` (tall) is dominant over `t` (short). Which statement is correct?

Its genotype is tall and its phenotype is `Tt`.
Its genotype is `Tt` and its phenotype is tall.
Its genotype is short and its phenotype is `tt`.
Its genotype and phenotype are both `Tt`.

Show Answer

Answer: B) Its genotype is `Tt` and its phenotype is tall.

`Tt` describes the **genotype** (the alleles). Because `T` is dominant, the plant's **phenotype** is tall. Phenotype is described with words like 'tall' or 'short', not with allele letters.

Check Understanding: Dominant and Recessive

Use your knowledge of dominant and recessive alleles to answer.

In a simple dominant–recessive trait, which genotype will always show the recessive phenotype?

Hh
HH
hh
Either HH or Hh